Ginkgo biloba L., the last surviving member of a family of trees (Ginkgoacea) that appeared more than 250 million years ago, has been mentioned in the Chinese Materia Medica for more than 2,500 years (Drieu, 2000). A number of G. biloba natural products have been isolated (Hasler, 2000), the most unique being the terpene trilactones, i.e. ginkgolides A, B, C, J and M (1–5) and bilobalide (6) (FIG. 1) (Nakanishi, 1967; Okabe, 1967; Nakanishi, 1971; Weinges, 1987). The ginkgolides are diterpenes with an aesthetic cage skeleton consisting of six 5-membered rings, i.e., a spiro[4.4]nonane carbocycle, three lactones and a tetrahydrofuran. The difference between the five ginkgolides lies in the variation in the number and positions of hydroxyl groups on the spirononane framework (FIG. 1).
A standardized G. biloba extract (EGb 761) containing terpene trilactones (5–7%) and flavonoids (22–24%) has demonstrated neuromodulatory properties (DeFeudis, 2000), and several clinical studies using EGb 761 have reported positive effects on various neurodegenerative diseases (Logani, 2000; Oken, 1998; Kleijnen, 1992; Søholm, 1998; Diamond, 2000; van Dongen, 2000), including Alzheimer's disease (AD). In two studies involving a total of 549 AD patients, EGb 761 significantly slowed the loss of cognitive symptoms of dementia, with an efficacy in between donezepil (Aricept®) and rivastigmine (Exelon®), the two currently marketed drugs for treatment of AD symptoms (Le Bars, 1997; Kanowski, 1996). Moreover, a recent study by Schultz and co-workers found that EGb 761 upregulated several genes in rat hippocampus and cortex, including genes expressing proteins such as transthyretin and neuronal tyrosine/threonine phosphatase, both of which are believed to be involved in AD (Watanabe, 2001). Several recent studies on healthy volunteers have shown positive effects of EGb 761 on short-term working memory (Kennedy, 2000; Polich, 2001; Rigney, 1999; Stough, 2001) indicating that constituents of G. biloba also influence the brain under physiological conditions.
Although the molecular basis for the action of G. biloba terpene trilactone constituents on the central nervous system (CNS) is only poorly understood, it is known that the ginkgolides, particularly ginkgolide B (GB, 2), is a potent in vitro antagonist of the platelet-activating factor receptor (PAFR) (Braquet, 1985).
A number of G. biloba constituents have been isolated, including the unique terpene trilactones, i.e., ginkgolides A, B, C, J and M and bilobalide (Nakanishi, 1967; Okabe, 1967; Nakanishi, 1971; Weinges, 1987). Ginkgolides are diterpenes with a cage skeleton consisting of six 5-membered rings, the difference between the five ginkgolides being in the variation in the number and positions of hydroxyl groups on the spirononane framework.
Although the molecular basis for the action of G. biloba terpene trilactone constituents in the central nervous system (CNS) is only poorly understood, it is known that ginkgolides, particularly ginkgolide B (GB, 1, FIG. 1), is a potent in vitro antagonist of the platelet-activating factor receptor (PAFR) (Braquet, 1987; Braquet, 1991). The PAFR is a potential target for neurodegenerative diseases (Singh, 2001) such as senile dementia (http://www.herbs.org/greenpapers/ginkgo.htm), stroke (Lindsberg, 1990) and nerve cell damage due to ischemia (Krieglstein, 1994).
PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, FIG. 2) is a phospholipid mediator involved in numerous disorders. PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine)has been suggested as a retrograde messenger in long-term potentiation (LTP) (Kato, 1994), thus indicating the importance of the PAFR as a target for ginkgolides. PAF has been implicated in a number of immunological, inflammatory and vascular disorders (Chung, 1995) including asthma (Nagase, 2002) and endotoxic shock (Tsuneyuki, 1996). In the latter case, PAFR antagonists have been shown to attenuate the effects of endotoxic shock in rats. Hyperacute rejections arising from PAF-associated reactions of either xenoperfusion (Cruzado, 1997) or renal transplants (Grino, 1994) have been found to be preventable by PAFR antagonists. In a separate study, PAFR antagonists significantly prevented pulmonary edema after myocardial ischemia in dogs (Taniguchi, 1992). The role PAF plays in a broad range of physiological conditions appears well-documented given the above examples, underscoring the importance of PAFR antagonists in inhibiting their undesirable effects.
These effects are manifested through binding of PAF to the PAFR, a G protein-coupled receptor that is found in organs such as the lungs, liver, kidneys (Ishii, 2000; Prescott, 2000; Shukla, 1996), and brain (Bito, 1992; Mori, 1996). The function of PAF in the brain is still not clear, although PAF has been suggested to play a role in diseases of aging (Kroegel, 1992), and in initiating HIV-related neuropathogenesis (Perry, 1998).
With few exceptions previous structure-activity relationship (SAR) studies of terpene trilactones on the PAFR have focused almost entirely on derivatives of GB (2). In all cases the derivatives were evaluated for their ability to prevent PAF-induced aggregation of rabbit platelets. Corey et al. investigated various intermediates encountered in the total syntheses of ginkgolide A (GA, 1) (Corey, 1988), GB (2) (Corey, 1988) and bilobalide (BB, 6) (Corey, 1987), and found that although the terminal methyl-bearing lactone was not essential for activity and could be replaced by other lipophilic groups (Corey, 1989), the tert. butyl group was important for PAFR antagonism (Corey, 1991). Park et al. synthesized over 200 derivatives of GB (2), with particular focus on aromatic substituents at 10-OH, and found most of them to be more potent than the parent compound (Park, 1996). Similar derivatives recently synthesized by Hu et al. also yielded compounds more potent than GB (2) (Hu, 1999; Hu, 2000), whereas other variations in GA (1) and GB (2) led to a decrease in activity (Hu, 2000; Hu, 2001).
However, none of the cited references disclose labeled analogs of ginkgolides useful for imaging studies. The following describes the preparation of a series of ginkgolide derivatives with photoactivatable groups and fluorescent groups, as well as groups that potentially can be radiolabeled with positron emitters such as 11C or 18F. These analogs, together with the native terpene trilactones (1–6), have been assessed for their ability to displace radioligand binding to cloned PAFR.